Glass composition for crystallized glass

ABSTRACT

A polished glass disk medium substrate suitable for use as a substrate for a hard disk, a hard disk containing the substrate and methods for making the substrate. The substrate containing glass forming raw materials may be formed so as to have a Young&#39;s modulus of 110 or higher.

RELATED APPLICATION

This application claims priority to Japanese Patent Application No.2000-100844 filed in Japan on Apr. 3, 2000, the contents of which arehereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a glass composition, and specificallyrelates to a glass composition suited for crystallized glass. Morespecifically, the present invention relates to a composition forcrystallized glass disk medium. Such disk medium include hard disks,magnetic disks, optical disks and magnetic-optical disks

DESCRIPTION OF THE PRIOR ART

Aluminum and glass are known materials suitable for use as magnetic disksubstrates. Among these substrates, glass substrates have been the focusof most attention due to their superior surface smoothness andmechanical strength. Such glass substrates include chemically reinforcedglass substrates strengthened by ion exchange on the surface, andcrystallized glass substrates having strengthened bonds by depositing acrystal component on the substrate.

The performance demands of recent substrates have become more severe dayby day, and improved performance is particularly sought regardingstrength, flex and warp during high-speed rotation. This type ofperformance can be expressed by the Young's modulus of the substratematerial, in which a higher numerical value is desirable.

For example, the composition disclosed in Japanese Laid-Open PatentApplication No. 11-322362 attains a Young's modulus value of 130 orgreater. However, this prior art requires extremely high thermalprocessing temperatures which complicate the manufacturing process, thatis, this art requires a primary processing temperature of 800° C., and asecondary processing temperature of 1,000° C.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved glasscomposition.

Another object of the present invention is to provide a glasscomposition having a high Young's modulus and which is highly suited formass production.

These objects are attained with a glass composition of the presentinvention desirably having the main components within the rangesdescribed below:

about 35 wt % or more, but less than about 50 wt % SiO₂;

about 5 wt % or more, but less than about 30 wt % Al₂O₃;

about 20 wt % or more, but less than about 22 wt % MgO; and

about 5 wt % or more, but less than about 12 wt % TiO₂.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiments of the present invention are describedhereinafter.

These objects are attained with a glass composition of the presentinvention desirably having the main components within the rangesdescribed below:

about 35 wt % or more, but less than about 50 wt % SiO₂;

about 5 wt % or more, but less than about 30 wt % Al₂O₃;

about 20 wt % or more, but less than about 22 wt % MgO; and

about 5 wt % or more, but less than about 12 wt % TiO₂.

When the composition content of SiO₂ used as a glass forming oxide isless than about 35 wt %, melting characteristics are typically adverselyaffected, and when the percentage exceeds about 50 wt %, a stabilizedstate of glass is achieved and crystal deposition typically becomesdifficult.

Aluminum oxide (Al₂O₃) is an intermediate oxide of glass, and is astructural component of the crystal-phase magnesium-aluminum crystalsformed during heating. When the composition content is less than about 5wt %, there are typically few crystals formed, and the desired strengthis not obtained, whereas when the percentage exceeds about 30 wt %, themelting temperature is typically raised and devitrification readilyoccurs.

Magnesium oxide (MgO) is a fluxing agent, which is added to induce thecrystal particles to nucleate and form crystal particle clusters. Whenthe composition content is less than about 20 wt %, the workingtemperature range is typically narrowed, and the chemical durability ofthe glass matrix phase is not typically improved. When the compositioncontent exceeds about 22 wt %, other crystal phase matter is oftendeposited and the desired strength is typically difficult to obtain.

Titanium oxide (TiO₂) is a crystal nucleating agent, which is often anessential component for magnesium silicate crystal deposition.Furthermore, TiO₂ functions as a fluxing agent to improve stabilityduring production. When the composition content is less than about 5 wt%, melting characteristics are typically adversely affected, and crystalgrowth is often difficult. When the content exceeds about 12 wt %,crystallization typically progresses rapidly, the crystallization stateoften becomes difficult to control, the deposited crystals are typicallycoarse with heterogeneity of the crystal phase, and a fine homogeneouscrystal structure often cannot be obtained, such that the requiredsurface smoothness for use as a disk substrate is difficult to obtain bya polishing process. Furthermore, devitrification readily occurs duringfusion molding, and mass production characteristics are reduced.

The manufacturing method is described below. The raw materials of theultimately produced glass substrate are thoroughly mixed in specificproportions, then introduced to a platinum crucible and melted. Aftermelting, the melted material is poured into a mold to form anapproximate shape. Then the material is annealed to room temperature.Next, the material is maintained at a primary heating processtemperature of about 500 to about 680° C. during a primary process(heating process) to generate crystal nuclei. Then, the material ismaintained at a secondary heating process temperature of about 680 toabout 800° C. during a secondary process to grow crystal nuclei. Thenthe material is cooled to obtain the crystallized glass.

This material may be used as a disk substrate by processing such aspolishing to attain a desired shape and thickness.

By using the above raw materials and the process described herein, anextremely high Young's modulus and high mass production characteristicsare obtainable. Even higher performance is obtained by adding thecomponents described below in a suitable range.

Stability during manufacture is improved by the addition of Li₂O, whichfunctions as a fluxing agent. When the composition content is less thanabout 0.1 wt %, there is inadequate improvement in meltingcharacteristics. When the composition content exceeds about 8 wt %,stability often decreases during the polishing and washing processes.

Zinc oxide (ZnO) functions as a fluxing agent which augments uniformcrystal deposition. When the composition content is less than about 0.1wt %, there is typically insufficient improvement in crystalhomogeneity. When the composition content exceeds about 22 wt %, theglass becomes stable, and crystallization is suppressed, such that thedesired strength is often difficult to obtain.

Phosphoric anhydride (P₂O₅), which functions as a fluxing agent, is anucleating agent for depositing silicate crystals, and is an importantcomponent for uniform deposition of crystals on the entirety of theglass. When the composition content is less than about 0.1 wt %,sufficient formation of crystal nuclei typically becomes difficult,crystal particles are often coarse, heterogeneous crystal depositionoften occurs, the desired fine homogeneous crystal structure may bedifficult to obtain, such that the required surface smoothness for useas a disk substrate may be difficult to obtain by a polishing process.When the content exceeds about 5.0 wt %, reactivity to the filter mediumincreases during melting, and devitrification increases so as to reducemass production characteristics during fusion molding. Chemicaldurability typically decreases, there is concern that the magnetic layermay be affected, and stability is often reduced during the polishing andwashing processes.

Adding ZrO₂ which functions as a glass modifying oxidant also functionseffectively as a glass crystal nucleating agent. When the content ratiois less than about 0.1 wt %, sufficient formation of crystal nucleitypically becomes difficult, crystal particles are often coarse,heterogeneous crystal deposition often occurs, the desired finehomogeneous crystal structure may be difficult to obtain, such that therequired surface smoothness for use as a disk substrate may be difficultto obtain by a polishing process. Furthermore, chemical durability andmigration resistance are often reduced, there is concern that themagnetic layer may be affected, and stability is often reduced duringthe polishing and washing processes. When the content exceeds about 12wt %, the melting temperature is raised, devitrification readily occurs,and fusion molding typically becomes difficult. Furthermore, thedeposition crystal phase fluctuates such that desired characteristicsare often difficult to obtain.

The addition of CaO, which functions as a fluxing agent, supplementsuniform crystal deposition. When the composition content is less thanabout 0.1 wt %, sufficient improvement in crystal homogeneity is nottypically obtained. When the content exceeds about 9 wt %, chemicaldurability is not typically improved.

Crystal nucleating material is increased by the addition of Nb₂O₅, whichworks as a fluxing agent. When the composition content is less thanabout 0.1 wt %, there is often inadequate rigidity improvement. When thecomposition content exceeds about 9 wt %, crystallization of the glasstypically becomes unstable, the deposition crystal phase typicallybecomes uncontrollable, and the desired characteristics are oftendifficult to obtain.

The addition of Ta₂O₅, which works as a fluxing agent, improves fusionand strength, and also improves chemical durability in the glass matrixphase. When the composition content is less than about 0.1 wt %, thereis typically inadequate rigidity improvement. When the compositioncontent exceeds about 9 wt %, crystallization of the glass typicallybecomes unstable, the deposition crystal phase becomes uncontrollable,and the desired characteristics are often difficult to obtain.

Stability during manufacture is improved by the addition of K₂O, whichfunctions as a fluxing agent. When the composition content is less thanabout 0.1 wt %, there is inadequate improvement in meltingcharacteristics. When the composition content exceeds about 9 wt %, theglass typically becomes stable and crystallization is suppressed,chemical durability is often reduced, and there is concern that themagnetic layer will be affected, and stability often decreases duringthe polishing and washing processes.

Glass phase splitting is promoted by adding B₂O₃, which works as aformer, and accelerates crystal deposition and growth. When thecomposition content is less than about 0.1 wt %, improvement of meltingcharacteristics is typically inadequate. When the composition contentexceeds about 9 wt %, glass devitrification readily occurs, moldingtypically becomes difficult, and the crystals often become coarse suchthat fine crystals is difficult to obtain.

Rigidity is improved by adding Y₂O₃, which functions as a fluxing agent.When the composition content is less than about 0.1 wt %, there istypically inadequate rigidity improvement. When the composition contentexceeds about 9 wt %, crystal deposition is often suppressed, sufficientcrystallization is difficult to obtain, and desired characteristics areoften not attained.

Stability during mass production is improved by adding Sb₂O₃, whichfunctions as a clarifier. When the composition content is less thanabout 0.1 wt %, there is typically insufficient clarification effect,and production characteristics are typically reduced. When thecomposition content exceeds about 9 wt %, crystallization of the glassoften becomes unstable, the deposition crystal phase typically becomesuncontrollable, and the desired characteristics are often difficult toobtain.

Stability during production is improved by adding As₂O₃, which functionsas a clarifier. When the composition content is less than about 0.1 wt%, there is often insufficient clarification effect, and productioncharacteristics are often reduced. When the composition content exceedsabout 9 wt %, crystallization of the glass typically becomes unstable,the deposition crystal phase typically becomes uncontrollable, and thedesired characteristics are often difficult to obtain.

The glasses of the present invention may have one or more crystallinephases and an amorphous phase. The crystalline phases represent about 50to about 60 percent of the total glass composition. Preferredembodiments include a main crystalline phase of clinoenstatite whichdesirably represents at least about 80 percent by weight of the total ofall crystalline phases. Preferred embodiments may also include asecondary crystalline phase of, for example, enstatite magnesiumaluminum silicate, and/or zinc titanium oxide which desirably representsless than about 20 percent by weight of the total crystalline phase.

Although the present invention is described in detail in the followingexamples, the invention is not limited to these examples. Tables 1-4show the glass composition in percent-by-weight of Examples 1-39. Glasssubstrates were obtained by the previously described manufacturingmethod in accordance with these numerical examples.

In the tables, C1 represents a crystal phase of clinoenstatite (MgSiO₃),C2 represents a crystal phase of enstatite (MgSiO₃), M1 represents acrystal phase of magnesium aluminum silicate {(Mg l)SiO₃}, Z1 representsa crystal phase of zinc titanium oxide (Zn₂Ti₃O₈) and Z2 represents acrystal phase of zinc titanium oxide (Zn₂TiO₄).

TABLE 1 Ex. 01 Ex. 02 Ex. 03 Ex. 04 Ex. 05 Ex. 06 Ex. 07 Ex. 08 Ex. 09Ex. 10 SiO₂ 45.0 45.0 45.0 45.0 45.0 45.0 43.0 37.0 43.0 41.0 Al₂O₃ 25.023.0 21.0 21.0 27.0 25.0 25.0 25.0 25.0 25.0 MgO 20.0 20.0 20.0 20.021.0 21.0 21.0 21.0 22.0 22.0 TiO₂ 5.0 7.0 9.0 12.0 5.0 7.0 9.0 12.0 5.07.0 Li₂O 5.0 5.0 5.0 2.0 2.0 2.0 2.0 5.0 5.0 5.0 Primary TreatmentTemperature (° C.) 660 660 660 660 660 660 660 660 660 660 SecondaryTreatment Temperature (° C.) 700 700 700 700 700 700 700 700 700 700Primary Treatment Time (hr) 5 5 5 5 5 5 5 5 5 5 Secondary TreatmentTemperature (hr) 5 5 5 5 5 5 5 5 5 5 Young's Modules (G Pa) 114.2 125.2133 153.4 115.2 126.2 129.2 131.9 111.9 118 Diameter of Crystal (nm) 3030 30 30 30 30 30 30 30 30 Main Crystal Phase C1 C1 C1 C1 C1 C1 C1 C1 C1C1 Secondary Crystal Phase C2 C2 C2 C2 C2 C2 C2 C2 C2 C2 Other CrystalPhase M1 M1 M1 M1 M1 M1 M1 M1 M1 M1

TABLE 2 Ex. 11 Ex. 12 Ex. 13 Ex. 14 Ex. 15 Ex. 16 Ex. 17 Ex. 18 Ex. 19Ex. 20 SiO₂ 39.0 45.0 43.0 37.0 43.0 37.0 43.0 37.0 43.0 37.0 Al₂O₃ 25.020.0 17.0 12.0 24.5 22.0 23.0 19.0 23.0 19.0 MgO 22.0 22.0 21.0 21.021.0 21.0 21.0 21.0 21.0 21.0 TiO₂ 9.0 12.0 9.0 12.0 9.0 12.0 9.0 12.09.0 12.0 Li₂O 5.0 1.0 2.0 5.0 2.0 5.0 2.0 5.0 2.0 5.0 ZnO 8.0 13.0 P₂O₅0.5 3.0 ZrO₂ 2.0 6.0 CaO 2.0 6.0 Primary Treatment Temperature (° C.)660 660 660 660 660 660 660 660 660 660 Secondary Treatment Temperature(° C.) 700 700 700 700 700 700 700 700 700 700 Primary Treatment Time(hr) 5 5 5 5 5 5 5 5 5 5 Secondary Treatment Temperature (hr) 5 5 5 5 55 5 5 5 5 Young's Modules (G Pa) 120.4 155.9 129.2 131.9 129.2 131.9129.2 131.9 129.2 131.9 Diameter of Crystal (nm) 30 30 30 30 30 30 30 3030 30 Main Crystal Phase C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 Secondary CrystalPhase C2 C2 C2 C2 C2 C2 C2 C2 C2 C2 Other Crystal Phase M1 M1 M1 M1 M1M1 M1 M1 M1 M1 Other Crystal Phase Z1 Z1 Other Crystal Phase Z2 Z2

TABLE 3 Ex. 21 Ex. 22 Ex. 23 Ex. 24 Ex. 25 Ex. 26 Ex. 27 Ex. 28 Ex. 29Ex. 30 SiO₂ 43.0 37.0 43.0 37.0 43.0 37.0 43.0 37.0 43.0 37.0 Al₂O₃ 23.020.0 23.0 20.0 24.8 21.0 23.0 19.0 23.0 21.0 MgO 21.0 21.0 21.0 21.021.0 21.0 21.0 21.0 21.0 21.0 TiO₂ 9.0 12.0 9.0 12.0 9.0 12.0 9.0 12.09.0 12.0 Li₂O 2.0 5.0 2.0 5.0 2.0 5.0 2.0 5.0 2.0 5.0 Nb₂O₅ 2.0 5.0Ta₂O₅ 2.0 5.0 K₂O 0.2 4.0 B₂O₃ 2.0 6.0 Y₂O₃ 2.0 4.0 Primary TreatmentTemperature (° C.) 660 660 660 660 660 660 660 660 660 660 SecondaryTreatment Temperature (° C.) 700 700 700 700 700 700 700 700 700 700Primary Treatment Time (hr) 5 5 5 5 5 5 5 5 5 5 Secondary TreatmentTemperature (hr) 5 5 5 5 5 5 5 5 5 5 Young's Modules (G Pa) 132.2 134.6130.7 134.6 129.2 131.9 129.2 131.9 132.2 134.6 Diameter of Crystal (nm)30 30 30 30 30 30 30 30 30 30 Main Crystal Phase C1 C1 C1 C1 C1 C1 C1 C1C1 C1 Secondary Crystal Phase C2 C2 C2 C2 C2 C2 C2 C2 C2 C2 OtherCrystal Phase M1 M1 M1 M1 M1 M1 M1 M1 M1 M1

TABLE 4 Ex. 31 Ex. 32 Ex. 33 Ex. 34 Ex. 35 Ex. 36 Ex. 37 Ex. 38 Ex. 39SiO₂ 43.0 37.0 43.0 37.0 43.0 37.0 43.0 37.0 47.0 Al₂O₃ 24.8 24.5 23.021.0 20.0 17.0 23.0 20.0 20.0 MgO 21.0 21.0 21.0 21.0 21.0 21.0 21.021.0 21.0 TiO₂ 9.0 12.0 9.0 12.0 9.0 12.0 9.0 12.0 12.0 Li₂O 2.0 5.0 2.05.0 2.0 5.0 2.0 5.0 Sb₂O₃ 0.2 0.5 2.0 4.0 5.0 8.0 As₂O₃ 2.0 5.0 PrimaryTreatment Temperature (° C.) 660 660 660 660 660 660 660 660 660Secondary Treatment Temperature (° C.) 700 700 700 700 700 700 700 700700 Primary Treatment Time (hr) 5 5 5 5 5 5 5 5 5 Secondary TreatmentTemperature (hr) 5 5 5 5 5 5 5 5 5 Young's Modules (G Pa) 129.2 131.9129.2 131.9 129.2 131.9 129.2 131.9 160.3 Diameter of Crystal (nm) 30 3030 30 30 30 30 30 30 Main Crystal Phase C1 C1 C1 C1 C1 C1 C1 C1 C1Secondary Crystal Phase C2 C2 C2 C2 C2 C2 C2 C2 C2 Other Crystal PhaseM1 M1 M1 M1 M1 M1 M1 M1 M1

The present invention provides a glass substrate having excellentproduction characteristics and a Young's modulus of 110 or higher.

Although the present invention has been fully described by way ofexamples with reference to the accompanying drawings, it is to be notedthat various changes and modification will be apparent to those skilledin the art. Therefore, unless such changes and modifications depart fromthe scope of the present invention, they should be construed as beingincluded therein.

What is claimed is:
 1. A polished glass disk medium substrate formed ofa mixture of glass forming raw materials comprising: about 35% to about50% by weight SiO₂; about 5% to about 30% by weight Al₂O₃; about 20% toabout 22% by weight MgO; about 5% to about 12% by weight TiO₂, and about0.1% to about 8% by weight Li₂O.
 2. The polished glass disk mediumsubstrate according to claim 1, wherein the raw materials furthercomprise about 0.1% to about 22% by weight ZnO.
 3. The polished glassdisk medium substrate according to claim 1, wherein the raw materialsfurther comprise about 0.1% to about 5% by weight P₂O₅.
 4. The polishedglass disk medium substrate according to claim 1, wherein the rawmaterials further comprise about 0.1% to about 12% by weight ZrO₂. 5.The polished glass disk medium substrate according to claim 1, whereinthe raw materials further comprise about 0.1% to about 9% by weight CaO.6. The polished glass disk medium substrate according to claim 1,wherein the raw materials further comprise about 0.1% to about 9% byweight Nb₂O₅.
 7. The polished glass disk medium substrate according toclaim 1, wherein the raw materials further comprise about 0.1% to about9% by weight Ta₂O₅.
 8. The polished glass disk medium substrateaccording to claim 1, wherein the raw materials further comprise about0.1% to about 9% by weight K₂O.
 9. The polished glass disk mediumsubstrate according to claim 1, wherein the raw materials furthercomprise about 0.1% to about 9% by weight B₂O₃.
 10. The polished glassdisk medium substrate according to claim 1, wherein the raw materialsfurther comprise about 0.1% to about 9% by weight Y₂O₃.
 11. The polishedglass disk medium substrate according to claim 1, wherein the rawmaterials further comprise about 0.1% to about 9% by weight Sb₂O₃. 12.The polished glass disk medium substrate according to claim 1, whereinthe raw materials further comprise about 0.1% to about 9% by weightAs₂O₃.
 13. The polished glass disk medium substrate according to claim1, said raw materials consisting essentially of: about 35% to about 50%by weight SiO₂; about 5% to about 30% by weight Al₂O₃; about 20% toabout 22% by weight MgO; about 5% to about 12% by weight TiO₂, and about0.1% to about 8% by weight of Li₂O.
 14. The polished glass disk mediumsubstrate according to claim 13, further containing one or more of thefollowing: about 0.1% to about 22% by weight ZnO; about 0.1% to about 5%by weight P₂O₅; about 0.1% to about 12% by weight ZrO₂; about 0.1% toabout 9% by weight CaO; about 0.1% to about 9% by weight Nb₂O₅; about0.1% to about 9% by weight Ta₂O₅; about 0.1% to about 9% by weight K₂O;about 0.1% to about 9% by weight B₂O₃; about 0.1% to about 9% by weightY₂O₃; about 0.1% to about 9% by weight Sb₂O₃; and about 0.1% to about 9%by weight As₂O₃.
 15. The polished glass disk medium substrate accordingto claim 12, wherein said substrate is essentially free of BaO, ZrO₂,B₂O₃ and NiO.
 16. The polished glass disk medium substrate according toclaim 1, comprising crystalline phases and amorphous phases.
 17. Thepolished glass disk medium substrate according to claim 16, wherein thecrystalline phases represent about 50 to about 60 percent by weight ofthe total glass composition.
 18. The polished glass disk mediumsubstrate according to claim 16, comprising a crystalline phase ofclinoenstatite.
 19. The polished glass disk medium substrate accordingto claim 18, wherein the crystalline phase of clinoenstatite representsat least about 80 percent by weight of the crystalline phases.
 20. Thepolished glass disk medium substrate according to claim 16, comprising acrystalline phase of enstatite.
 21. The polished glass disk mediumsubstrate according to claim 20, wherein the crystalline phase ofenstatite represents less than or equal to about 20 percent by weight ofthe crystalline phases.
 22. The polished glass disk medium substrateaccording to claim 16, comprising a crystalline phase of magnesiumaluminum silicate.
 23. The polished glass disk medium substrateaccording to claim 22, wherein the crystalline phase of magnesiumaluminum silicate represents less than or equal to about 20 percent byweight of the crystalline phases.
 24. The polished glass disk mediumsubstrate according to claim 16, comprising a crystalline phase ofZn₂Ti₃O₈.
 25. The polished glass disk medium substrate according toclaim 24, wherein the crystalline phase of Zn₂Ti₃O₈ represents less thanor equal to about 20 percent by weight of the crystalline phases. 26.The polished glass disk medium substrate according to claim 16,comprising a crystalline phase of Zn₂TiO₄.
 27. The polished glass diskmedium substrate according to claim 26, wherein the crystalline phase ofZn₂TiO₄ represents less than or equal to about 20 percent by weight ofthe crystalline phase.
 28. The polished glass disk medium substrateaccording to claim 1, comprising a main crystalline phase ofclinoenstatite and a secondary crystalline phase of enstatite.
 29. Thepolished glass disk medium substrate according to claim 28, furthercomprising one or more of a crystalline phase of magnesium aluminumsilicate, a crystalline phase of Zn₂Ti₃O₈ and a crystalline phase ofZn₂TiO₄.
 30. The polished glass disk medium substrate according to claim1, wherein said glass substrate has a Young's modulus of 110 or higher.31. The polished glass disk medium substrate according to claim 1,wherein said substrate is prepared by heating glass forming rawmaterials to a temperature, T₁, between about 500 and 680° C. togenerate crystal nuclei; heating at a temperature, T₂, between about 680and about 800° C. to grow crystal nuclei; and cooling to obtaincrystallized glass.
 32. A recording disk comprising the polished glassdisk medium substrate defined in claim
 1. 33. The recording diskaccording to claim 32, wherein said recording disk is a hard disk. 34.The recording disk according to claim 32, wherein said recording disk isa magnetic disk.
 35. The recording disk according to claim 32, whereinsaid recording disk is an optical disk.
 36. The recording disk accordingto claim 32, wherein said recording disk is a magnetic-optical disk. 37.A method of making a glass disk medium substrate comprising: heatingglass forming raw materials to a temperature sufficiently high to meltthe raw materials; forming a disk medium substrate; and crystallizingthe disk medium substrate, wherein said crystallizing comprises heatingthe disk medium substrate to a temperature, T₁, between about 500 and680° C. to generate crystal nuclei; heating at a temperature, T₂,between about 680 and about 800° C. to grow crystal nuclei; and coolingto obtain crystalized glass, wherein the glass forming raw materialscomprise: about 35% to about 50% by weight SiO₂; about 5% to about 30%by weight Al₂O₃; about 20% to about 22% by weight MgO: about 5% to about12% by weight TiO₂, and about 0.1% to about 8 by weight of Li₂O.
 38. Themethod according to claim 37, further comprising polishing said glassdisk medium substrate.
 39. The method according to claim 37, whereinsaid glass disk medium substrate formed has a Young's modulus of 110 orhigher.